Topic 2. Data Transmission

Question 1

What is the structure of a data packet?

Answer 1

Packet Header
Payload
Trailer

Question 2

What does the packet header contains?

Answer 2

The destination IP Address
Packet number
Sender’s address

Question 3

What does the payload contain?

Answer 3

The actual chunk of data being transmitted

Question 4

What is the process of packet switching?

Answer 4

1. Data is broken down into packets
2. Each packet is given a sequence number and sent on its way
3. Each packet may take a different route to the next depending on the quickest route at the time
4. Routers control the route that a packet takes
5. Packets may arrive out of order. Once the last packet arrived, the sequence numbers are used to put them back into order. The data can then be reconstructed correftly

Question 5

What is serial transmission?

Answer 5

Data bits are sent one at a time down one cable
Very high data speeds can be achieved
Used with USB (Universal Serial Bus) interface

Question 6

What are the advantages of serial transmission?

Answer 6

Lower interference
More reliable over long distances
Smaller, simpler and cheaper connections to produce

Question 7

What is the disadvantage of serial transmission?

Answer 7

Lower transmission speeds

Question 8

What is parallel transmission?

Answer 8

Several bits are sent simultaneously
Used with printer cables or internal device connections

Question 9

What are the advantages of parallel transmission?

Answer 9

Faster than serial transmission
Can send data in both directions at the same time

Question 10

What are the disadvantages of parallel transmission?

Answer 10

Can only be used over very short distances
Interference issues between wires
Data corruption can occur over longer distances
Data bits can move at different speeds causing them to arrive out of sync known as skewing.

Question 11

What is simplex transmission?

Answer 11

Data travels in one direction only down a single cable

Question 12

What can you use simplex transmission for?

Answer 12

Sending data from the computer to a monitor

Question 13

What is half-duplex transmission?

Answer 13

Data can travel in both directions along a single cable but not simultaneously

Question 14

What can you use half-duplex transmission for?

Answer 14

Used in parallel printer cables

Question 15

What is duplex transmission?

Answer 15

Data can travel in both directions simultaneously using two communication channels

Question 16

What can you use duplex transmission for?

Answer 16

Networking or Internet Cables to transmit and receive data at the same time.?

Question 17

How do parity checks detect errors in data transmission?

Answer 17

When sending a byte of data, one bit is used as the parity bit
Set to either 0 or 1 at the start to make the total numbers of 1s odd or even
The 1s are counted at the receiving end and if the wrong number of bits are 1 an error is detected and the data is resent

Question 18

How do parity block checks detect errors in data transmission?

Answer 18

A parity block check is used when sending multiple bytes in one transmission block.
Each byte will have its own parity bit
A final parity byte is used which contains a parity bit for each column
An error in the transmission can be detected but the exact position can be identified and corrected

Question 19

How do checksums detect errors in data transmission?

Answer 19

A checksum is calculated using an algorithm to sum all of the data to be sent in a transmission
The checksum is sent with the transmission and recalculated using the same method
If the starry and end sums do not match, the data has changed during transmission

Question 20

How does an echo check detect errors in data transmission?

Answer 20

An echo check is performed by asking the recipient computer to send back an exact copy of the data it has just received to the originator
The data is compared with what was originally sent
If both copies match, the data is deemed to have arrived without error

Question 21

How does ARQs detect errors in data transmission?

Answer 21

ARQs will automatically send back a positive acknowledgement if a data packet is deemed correct or they will ask for the data to be resent if an error has been detected
If no acknowledgement is returned within a certain time period, a timeout is registered and the sending computer resends the packet.
This process continues until the packet received is correct or until the ARQ limit is reached

Question 22

How do check digits detect errors in data transmission?

Answer 22

The check digit is an additional digit at the end of string of other numbers (commonly used in barcodes and ISBN numbers)
A check digit is recalculated on the data entry
If it matches with the one at the end of the number, the numbers are deemed correct

Question 23

How to calculate a check digit for an ISBN number

Answer 23

Multiply the ISBN numbers and the weight together
Add all the togethers
Find the remainder when the sum is divided by 10
Subtract the remainder from 10

Question 24

What is symmetric encryption?

Answer 24

Symmetric encryption (private key encryption) uses the same pre-shared key to encode and decode data
This means that the key must also be transferred to the recipient so that they can decrypt it

Question 25

What is asymmetric encryption?

Answer 25

Asymmetric encryption (public key encryption) uses two separate keys but related keys
A public key is shared with others and used by them to encrypt a message so it can only be deciphered by the intended recipient with their private key
(A private key must never be shared)